An inadvertently amusing headline appeared on the front page of the Times yesterday. "Help yourselves, Cameron tells the fat and the poor," suggesting that the Tory leader had been generously nodding the overweight towards the buffet table, encouraging them to load up with seconds. Given that this is clearly the week for gastronomic advice from our senior politicians, what with Gordon Brown urging us to toast our stale bread, this seemed like a sunny Conservative response to the current food crisis, Cameron's munificence surely designed to contrast with Labour austerity.

But no, that was not his meaning. As an amended, online version of the Times headline later made clear, Cameron's message - delivered in a speech to launch the Tories' byelection campaign in Glasgow East - was "take responsibility". It was part of a daring push into the perilous terrain of morality, with the Conservative leader arguing that politicians needed to speak once more of good and bad, right and wrong. Fear of seeming judgmental had led, he said, to the erosion "of responsibility, of social virtue, of self-discipline, respect for others". Society had been stripped of its morals, with no one willing to set boundaries between right and wrong.

Lofty talk but start with the base politics. Those thoughts on morality were offered just after Cameron had spoken about knife crime, now one of the electorate's chief concerns. In this, Cameron was following the lead set by Tony Blair, who believed that politicians have to talk about crime, a lot, to show that they care and that they're in touch with voters' fears. On this view, Blair had a consistent antagonist: Gordon Brown. Despite his coinage of "tough on crime, tough on the causes of crime", Brown's instinct was always to avoid dwelling on the topic lest one inflame people's fears yet further. Accordingly, he has made not so much as a single major speech on crime since he became prime minister.

The result is that Cameron is all over the issue of the hour, while Brown is telling us to eat our leftovers - and doing so as he sits down to a lavish, eight-course banquet of milk-fed lamb and pickled conger eel at the Hokkaido G8. Who looks out of touch?

Cameron's larger diagnosis of Britain as a "broken society" can also be understood as politics as usual. Oppositions facing long-serving governments always try to persuade the electorate that the country is going to the dogs, that there is a spiral of decline and decay that only their arrival in power can arrest. Blair played the same game in the 1990s, even as shadow home secretary, articulating public angst more clearly than the Major government. That memory prompts one Labourite to say that "Cameron is seeking his Jamie Bulger moment".

Of course it is an exaggeration to say that Britain is broken now, just as it was hyperbolic to claim the nation was in ruins 15 years ago. But that's a hard case for a government to make without sounding complacent. Witness the fate of junior minister Tom Harris who was slapped down last month when he told us to buck up and enjoy a prosperity that would have "made our parents gasp". He was right, just as it's right to note that the number of young people falling victim to violent crime appears to be static rather than on the rise - albeit with an apparent shift from guns to knives - but incumbent politicians say so at their peril.

All of which gave Cameron a hospitable climate for his speech on Monday. Even so, it was still a mark of his confidence that he gave it: he surely would not have dared a year ago. For Cameron took the risk of sounding like Richard Littlejohn or a Daily Mail editorial, berating a political correctness culture that doesn't blame a fat person for eating too much but casts them as a passive victim, "at risk of obesity" - a phrase Cameron singled out for mockery. This was the kind of man-in-the-pub talk that sank William Hague, and the early model Cameron would have gone nowhere near it. That he now has, and with such gusto, suggests he really does believe the Conservative brand has been sufficiently decontaminated that the party leader can now move on to the turf of the populist right without anxiety.

It is not without risk. Plenty on the centre and left, those Cameron has been so eager to reassure these last three years, will hear in his words not only a white-van man intolerance for PC mushiness, but also a return to hard-faced, Tebbitite impatience with the poor. Let's be clear: it was not just the chubby whom Cameron said had themselves to blame. "We talk about people being at risk of poverty, or social exclusion," he said, "as if these things ... are purely external events like a plague or bad weather ... But social problems are often the consequence of the choices that people make."

Cameron is gambling here: first that some centrist waverers won't be put off; and second, that some of the more unreconstructed members of his own party won't take that as a nod and a wink in their direction. The social commentator Richard Reeves - who welcomes Cameron's approach, has discussed it with the Tory inner circle, and presents a Radio 4 Analysis programme on morality and character tomorrow - can see that danger: "The Monday Club will be popping the champagne corks."

Nevertheless, the Tory leader is calculating that the potential benefits outweigh the risks. For one thing, he gets to pose as the teller of hard truths, always a neat stance for a politician. More importantly, he knows his message will resonate. Everyone can tell an anecdote of a teacher who has quite legitimately disciplined a child, only to be berated - or worse - by a parent who seems oblivious to right and wrong. Or of a bureaucrat who refuses to do the right thing, because to do so might be perceived as "judgmental".

Plenty will nod along with Cameron, in middle Britain and beyond, including those on low incomes. To their credit, the Blairites used to have several people who could speak to that same coalition with the same message of old-fashioned morality: Blair himself, but also the likes of David Blunkett and John Reid. It's hard to see anyone around today's cabinet table who can press a similar button.

But this latest move by Cameron includes one step that departs entirely from the conventional political playbook. Having diagnosed a broken society, the Tory leader goes out of his way to say he cannot solve it. There is no top-down, governmental remedy, he says. Only society can heal itself.

It may well be true that Britain's historic error has been to look to governments for answers to every problem. But Cameron is not a mere analyst; he is on course, according to the polls, to be Britain's next prime minister. Which makes this the biggest risk of all. The very act of diagnosing such a grave illness - a broken society - sets up the expectation that he will bring a cure, even as he insists he can do no such thing.

Voters will start asking what he plans to do about this malaise, what steps he will take on day one. What if the gentle nudges from the top don't work? Can he really be proposing that Downing Street be reduced to the "bully pulpit" once invoked by Teddy Roosevelt, a site for mere exhortation? His supporters will have to hope not. Because the British electorate expect rather more of their prime ministers - as David Cameron may one day find out.